Wide band variable transmittance optical device

ABSTRACT

We describe a wide band variable transmission optical device having an electronically active cell comprising a guest-host mixture of a liquid crystal host and a dichroic guest dye material contained between a pair of plastic substrates. The liquid crystal host has an axis orientation that is alterable between a clear state orientation and a dark state orientation and the dichroic guest dye material includes one or more dichroic dyes. The optical device is characterized in that it exhibits a wide absorption band that is greater than 175 nm within a visible wavelength range of 400-700 nm, has a clear state transmission equal to or above 30% and a dark state transmission equal to or below 40%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and any other benefit of, U.S.patent application Ser. No. 13/877,508, filed Jun. 13, 2013 entitledWIDE BAND VARIABLE TRANSMITTANCE OPTICAL DEVICE AND MIXTURE, whichitself claims priority of International Patent Application No.PCT/US2011/054701, filed Oct. 4, 2011, which claims priority of U.S.Provisional Patent Application Ser. No. 61/389,444, entitled WIDE BANDVARIABLE TRANSMITTANCE OPTICAL DEVICE AND MIXTURE, filed Oct. 4, 2010,the entire disclosures of which are fully incorporated herein byreference.

BACKGROUND

Variable transmission eyewear devices (glasses, goggles, visors, etc.)that can quickly change between a high-transmission “clear” state and alow-transmission “dark” state have many advantages over fixedtransmission eyewear and are highly desirable. An especially usefulfeature is the ability to make this quick change occur on demand,whether manually, at the touch of a button by the wearer, orautomatically, under the control of a light sensor and an electroniccircuit.

Previous attempts to create on-demand variable transmission eyewear haveemployed a variety of liquid crystal systems, using the ability ofliquid crystal molecules to alter their orientation with the applicationof an external electric field.

Liquid crystal devices can be broadly categorized as polarizer-based(which contain, at least, one polarizer) or guest-host systems. Thepolarizer-based systems are used in applications where the dark statetransmission is the most important parameter. In particular, they areused when it is necessary to obtain minimal transmission (nearly zero)conditions. Such applications include flat panel displays as well aswelding helmets and 3D glasses. However, polarizers limit the amount oflight transmission of the device, often well below a theoretical limitof only 50% transmission. Guest-host systems, on the other hand, weretraditionally used for display applications where the wide viewing angleand/or true color saturation were important. Examples include cockpitdisplays that allow the pilot and co-pilot to observe the same image.Guest-host systems are better suited for eyewear devices since theyallow for potential light transmission levels above 50%. In fact, somepatents (see e.g. Palffy-Muhoray et al., U.S. Pat. No. 6,239,778, IssuedMay 29, 2001) suggest use of guest-host for eyewear applications whereinthe guest-host device is comprised of a mixture of a liquid crystal“host” and a dichroic dye “guest” contained between a pair ofsubstrates. The liquid crystal “host” includes non-polarizing liquidcrystal material having an axis of orientation that is alterable byadjustment of voltage applied across the substrates that can changebetween a clear-state orientation and a dark-state orientationperpendicular thereto. The dye “guest” mixture comprises dichroic dyeswhich are dissolved within the liquid crystal host and which align withthe orientation of the liquid crystal material. A commercial embodimentof this approach is the Magic™ Ski Goggles, which uses a guest-hostliquid crystal system and plastic substrates (Park et al., U.S. Pat. No.7,567,306, issued Jul. 28, 2009).

Generally, it is desirable for such variable transmittance liquidcrystal optical devices to have good optical properties while usingplastic substrates, to exhibit a wide transmission swing (a widedifference between the clear and the dark states) and to absorb lightacross the broadest possible band in order to minimize the difference incolor between the clear and dark states.

There is however no current commercial liquid crystal guest-host devicethat provides wideband absorption greater than 175 nm for eyewearapplications. This is because knowledge relating to the use ofguest-host systems has been based on previous knowledge of displayapplications, which provide no guidance as to the parameters necessaryto make a successful wideband eyewear device. Liquid crystal displayshave very different performance requirements than eyewear devices. Forexample, liquid crystal displays traditionally use glass substrates,whereas optical devices suitable for eyewear are preferably plasticbased. Glass and plastic have very different properties; the sensitivityof the eye to certain parameters makes plastic products, which may beviable for display applications, unacceptable for eyewear applications.For example, non-uniformity in the visual distortions is paramount ineyewear applications. As such, traditional display materials orconfigurations are rendered unacceptable for eyewear applications.

Analytical parameters used to characterize guest-host systems includeabsorption spectrum, order parameter of the mixture, type of dielectricanisotropy (positive or negative), and the nematic to isotropictemperature (T_(NI)) of the liquid crystal-guest mixture (B. Bahadur.“Dichroic liquid crystal displays.” In Liquid Crystals—Applications andUses (Vol. 3) (pp. 65-208), 1992, Singapore: World Scientific). Inaddition, a device can be characterized by the type of substrate andthickness used, liquid crystal alignment in absence of an electricalfield, thickness of the cell, the swing in the transmission, opticaldistortion, and the cell gap of the cell as well as the pitch of achiral liquid crystal and the “thickness to pitch” ratio (d/p) of themixture. The performance of any device is dictated by the choice ofthese parameters, which themselves are inter-related. But, there is noanalysis or description of the exact nature of the interplay between theparameters in commercial guest-host devices.

For example, one difficulty in defining any parameters for eyewearapplications has been the inherent conflict between the properties ofvarious components used in a guest-host system. This can lead to aperceived physical limitation on the performance. For example,Applicants have found that a large transmission swing between the clearstate transmission and the dark state transmission is achieved by usinghigh performing dichroic dyes. However, such dyes have intrinsicallylower solubility, can disrupt liquid crystalline phase, and alter thenematic to isotropic phase transition temperature, T_(NI). Furthermore,such dyes dictate a higher degree of polarization dependence in theperformance. “Polarization dependence” is a measure of a material'sresponse to two orthogonal polarizations; i.e. where the opticalproperties of a material experienced by an incident light (such as indexof refraction or absorption/transmittance) are dependent on thepolarization of the incident light. An increase in polarizationdependence can in turn reduce the swing in the transmission between theclear and dark states. Furthermore, this property may also becomeundesirable because a higher polarization dependence can reveal evensmall structural imperfections within the liquid crystal cellconfiguration and/or any plastic substrates used for the technology.Since the eye can easily pick out even minor variations in the field ofvision, traditional systems using high performing dyes had poor opticalperformance.

The polarization dependence of a chiral nematic guest-host systemdepends upon how tightly twisted the liquid crystal molecules are (i.e.their “pitch”) in relation to the thickness of the liquid crystal layer.This ratio is measured by a parameter known as “thickness to pitchratio” or “d/p”. It is known that the larger the d/p of a liquid crystalmixture, the less polarization dependent it is. For example, one way toreduce the polarization dependence of a device is to use a liquidcrystal mixture with short pitch (<3 micron) in a thin cell (<3 microns)and a twisted structure with a thickness to pitch ratio (d/p) of >0.9.However, in addition to the fact that this makes manufacturing extremelydifficult and has prevented production, using 3 micron cell rather thanthicker cells can also reduce the clear and dark state transmission dueto the surface alignment effect, in which the liquid crystal moleculesare less responsive to an applied electric field because of theproximity of the two surfaces. This, in turn, can lead to a smallertransmission swing and hence a reduction in performance.

An approach to circumvent this obstacle is to trick the eye into notseeing imperfections in the cell or the plastic substrates. This can bedone if the device exhibits strong color dependence in the absorptionspectra. In other words, to avoid the eye seeing these imperfections, aguest-host system with strong color (i.e. a narrow absorption spectrumof <150 nm) is used. But such devices are limited in their transmissionswing and/or have a relatively narrow absorption band. As such, they donot fulfill the need for a wide band optical device.

Therefore, there is still a need for a variable transmittance liquidcrystal optical device with good optical properties that uses plasticsubstrates, exhibits a wide transmission swing and has a broadabsorption band (>175 nm).

We have discovered, and describe herein, a set of material and systemparameters, based on physical characteristics, and device configurationsthat can circumvent these obstacles and can accomplish the desiredsystem requirements described above.

SUMMARY OF INVENTION

Disclosed herein are variable transmission optical devices and methodsof making the same. Each optical device includes an electronicallyactive cell (referred to herein as “cell”) comprising a guest-hostmixture of a liquid crystal host and a dichroic guest dye materialcontained between a pair of plastic substrates. The liquid crystal hosthas an axis orientation that is alterable between a clear stateorientation and a dark state orientation perpendicular thereto. Thedichroic guest dye material comprises one or more dichroic dyes. Theelectronically active cell does not use polarizers.

The optical device may further comprise an electronically passive layerfor further reducing the transmission of light through the opticaldevice. The electronically passive layer may be a polarizationindependent absorptive, reflective or scattering layer, or apolarization dependent absorptive or reflective layer, or anycombination of the aforementioned layers. Examples of polarizationindependent layers are known in the art and include, but are not limitedto, a layer having a dye (tinted layer) or a photon-activatedphotochromic dye, a partially silvered mirror, an anti-glare plasticlayer, etc. Examples of polarization dependent layers are well-known andinclude, but are not limited to, absorptive or reflective polarizers, orlayers having a dichroic absorber such as a photochromic-dichroic dye.

In some embodiments, the electronically active cell by itself exhibits awide absorption band that is greater than 175 nm within a visiblewavelength range of 400-700 nm, has a clear state transmission equal toor above 30% and a dark state transmission equal to or below 40%.

In some embodiments, the optical device as a whole exhibits a wideabsorption band that is greater than 175 nm within a visible wavelengthrange of 400-700 nm, has a clear state transmission equal to or above30% and a dark state transmission equal to or below 40%. In someembodiments, the clear state transmission may be equal to or above 35%,40%, 45%, 50%, 55%, 60%, 65% or 70%. In some embodiments, the dark statetransmission may be equal to or below 35%, 30%, 25%, 20%, 15%, 10% or5%.

In another aspect of the present invention, the optical device maycontain a second electronically active cell that contains a liquidcrystal—dye mixture for further reducing the transmission of lightthrough the optical device. This second electronically active cell willabsorb or scatter a portion of light in either the Off state, when novoltage is applied, or in the On state, in response to an appliedvoltage.

Several versions of this optical device are contemplated. For example,in one version, the first electronically active cell itself exhibits awide absorption band that is greater than 175 nm within a visiblewavelength range of 400-700 nm. In another version, the first and thesecond electronically active cells each exhibit a wide (greater than 175nm) absorption band within the wavelength range of 400-700 nm. In yetanother version, the first and the second electronically active cells incombination exhibit a wide (greater than 175 nm) absorption band withinthe wavelength range of 400-700 nm.

Similarly, in one version, the first electronically active cell itselfexhibits a clear state transmission equal to or above 30% and a darkstate transmission equal to or below 40%. In other embodiments, thecombination of the first and second electronically active cells exhibita clear state transmission equal to or above 30% and a dark statetransmission equal to or below 40%. In some embodiments, the clear statetransmission may be equal to or above 35%, 40%, 45%, 50%, 55%, 60%, 65%or 70%. In some embodiments, the dark state transmission may be equal toor below 35%, 30%, 25%, 20%, 15%, 10% or 5%.

In yet another aspect of the present invention, the optical device'selectronically active cell has a guest-host mixture that furthercontains an ionic element to induce dynamic scattering in the liquidcrystal host when a second voltage is applied to the cell. This opticaldevice will also exhibit a wide absorption band having an aggregate fullwidth at half maximum (A-FWHF) that is greater than 175 nm within avisible wavelength range of 400-700 nm, a dark state transmission equalto or below 40% and a transmission swing between the clear stateorientation and the dark state orientation greater than or equal to 30%.In some embodiments, the clear state transmission may be equal to orabove 35%, 40%, 45%, 50%, 55%, 60%, 65% or 70%. In some embodiments, thedark state transmission may be equal to or below 35%, 30%, 25%, 20%,15%, 10% or 5%.

The above mentioned optical devices will have the following additionalcharacteristics:

In some embodiments, the guest-host mixture has a liquid crystal hostwith a negative dielectric anisotropy, so that the mixture has a clearstate orientation when no voltage is applied to the cell and a darkstate orientation when a voltage is applied to the cell. In thisexample, the liquid crystal host includes a chiral nematic material witha thickness to pitch ratio (d/p) of less than 0.9 but greater than orequal to 0.25. In some examples, the liquid crystal host has a thicknessto pitch ratio (d/p) of less than 0.8 or less than 0.7.

In other embodiments, the guest-host mixture has a liquid crystal hostwith a positive dielectric anisotropy, so that the mixture has a darkstate orientation when no voltage is applied to the cell and a clearstate orientation when a voltage is applied to the cell. In thisversion, the liquid crystal host comprises a chiral nematic materialwith a thickness to pitch ratio (d/p) of less than 5 but greater than orequal to 1.75. In some examples, the d/p is less than or equal to 4 orless than or equal to 3.

In some embodiments, the absorption band of the optical device, orelectronically active cell(s) is greater than 180 nm, 185 nm, 190 nm,195 nm or 200 nm within a visible wavelength range of 400-700 nm.

The pair of plastic substrates in the electronically active cell exhibitan optical retardation with less than ±20% variation in uniformityacross the area of the device (or cell). In some examples, the plasticsubstrates have an optical retardation variation of less than ±15%, orless than ±10%.

The electronically active cell of each optical device has a cell gapgreater than 3 microns but less than 20 microns. In some examples, thecell gap is equal to or greater than 5 microns but less than 15 microns.

The transmission swing of each electronically active cell is greaterthan or equal to 30%. In other examples, the transmission swing isgreater than or equal to of 35%, 40%, 45%, 50%, 55% or 60%.

The guest-host mixture of each optical device has a nematic-isotropictransition temperature T_(NI) greater than 40° C. In some examples, theT_(NI) is greater than 45° C., 50° C., 55° C., 60° C., 65° C., 70° C.,75° C., 80°, 85°, 90°, 100° or 110° C.

The guest-host mixture of each electronically active cell has an orderparameter, S_(mix), greater than 0.78. In other embodiments, theguest-host mixture has an order parameter greater than or equal to 0.79.In yet other embodiments, the guest-host mixture has an order parametergreater than or equal to 0.8.

In some embodiments, the guest-host mixture has a dichroic ratio D_(mix)greater than 11.5. In other embodiments, the dichroic ratio D_(mix) isgreater than 12, or greater than 12.5, or greater than 13.

The guest-host mixture includes one or more dichroic dyes in thedichroic guest dye material which are azo-based dyes having at least twoazo groups. In some embodiments, the dichroic dyes have 2-6 azo groups.In other embodiments, the dichroic dyes have 2-10 azo groups.

Any of the optical devices mentioned herein may be planar, or may becurved in at least one dimension.

Also disclosed herein are wide band mixtures for use in any of theoptical devices described above and methods of making the same.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other features and advantages of the present invention willbecome better understood with regard to the following description,appended claims, and accompanying drawings wherein:

FIGS. 1A and 1B are enlarged schematic cross-sectional representationsof a cell according to one version of the present invention. FIGS. 1C-1Eare schematic diagrams of the guest-host molecule orientation duringvarious states.

FIGS. 2A-2E are examples of graphs showing light absorption of differentcells or mixtures over a range of wavelengths.

FIG. 3 is a graph showing the absorbance spectra of a wideband dichroicdye. This graph is used in calculation of the dichroic ratio and orderparameter of the mixture.

FIG. 4 is a graph showing the transmission spectra of Example 1 in theOff (top line) and On state (bottom line).

FIG. 5 is a graph showing the transmission spectra of Example 2 in theOff (top line) and On (bottom line) states.

FIGS. 6A-6B are graphs showing the absorption curve and the transmissionspectra, respectively, of Example 3 in the Off and On states.

FIGS. 7A-7B are graphs showing the absorption curve and the transmissionspectra, respectively, of Example 4 in the Off and On states.

FIG. 8 shows enlarged schematic cross-sectional representations of adouble cell according to the concepts of the present invention.

DETAILED DESCRIPTION

Disclosed herein are electro-optical variable transmission opticaldevices. Each optical device includes an electronically active cell(referred to herein as “cell”) comprising a guest-host mixture of aliquid crystal host and a dichroic guest dye material contained betweena pair of plastic substrates. The liquid crystal host has an axisorientation that is alterable between a clear state orientation and adark state orientation perpendicular thereto. The electronically activecell does not use polarizers. The optical device or the electronicallyactive cell is characterized in that it exhibits a wide absorption band(>175 nm within a visible wavelength range of 400-700 nm) or neutraltint, has a clear state transmission equal to or above 30% and a wide(>30%) transmission swing (high contrast between clear and dark states).Also described are mixtures for use in such devices, referred to hereinas “wide band” mixtures; and methods of making said wide band mixturesand devices.

In some embodiments, in addition to the electronically active cell, theoptical device may further have an electronically passive layer forfurther reducing the transmission of light through the optical device.

Alternatively, the optical device may contain a second electronicallyactive cell that contains a liquid crystal—dye mixture for furtherreducing the transmission of light through the optical device byabsorbing or scattering light. This optical device is referred to as a“double stack”.

In another version, the optical device's electronically active cell hasa guest-host mixture that further contains an ionic element to inducedynamic scattering in the liquid crystal host when a second voltage isapplied to the cell.

Definitions

Unless specifically defined otherwise herein, the definitions foroptical parameters such as linear, circular and unpolarized light arethe same as those in “Principles of Optics Electromagnetic Theory ofPropagation, Interference and Diffraction of Light”, Max Born, et al.,Cambridge University Press; 7th edition (Oct. 13, 1999). Similarly, allliquid crystal terminology which is not specifically defined herein isto have the definition as used in Liquid Crystals Applications and Uses,vol. 3, edited by B. Bahadur, published by World Scientific PublishingCo. Pte. Ltd., 1992 (“Bahadur”).

“Absorption” as used herein is to define the percentage of light nottransmitted through the cell or optical device. It is related totransmission by the following: absorption=100%−transmission. Therefore,as used herein, “Absorption” refers to any mechanism stopping thetransmission of light through the device, including absorbance,reflectance and scattering. As used herein, “transmission” and“transmittance” are used interchangeably and mean the percentage oflight that is transmitted through a device (or cell).

“Clear state” refers to the state where the guest-host mixture exhibitsmaximal light transmittance.

“Dark state” refers to the state where the guest-host mixture exhibitsmaximal light absorption.

“Dichroic ratio”, “average dichroic ratio” or D_(mix) of the mixturerefers to the dichroic ratio of the guest-host mixture. The mixture maycontain one or more dyes as well as other dopants. The mixture dichroicratio may be measured using the formula for Effective Dichroic Ratio(D_(eff)) or Aggregate Effective Dichroic Ratio (D_(eff-agg)). Thus, asused herein, D_(mix), D_(eff) or D_(eff-agg) are used interchangeably(depending on which method is used to measure the dichroic ratio) anddescribe the same parameter.

“Electronically active” cell or layer refers to a cell or layer that canbe activated upon application of a voltage. Activation of such a cell orlayer will result in an alteration in the amount of light transmittancethrough the device.

“Narrow Band Absorption” as used herein, is defined as a spectralabsorption band width with an aggregate Full Width at Half Max (A-FWHM)that is less than 175 nm, where the entire spectral absorption band iscontained within 400 nm-700 nm.

Nematic-isotropic transition temperature or T_(NI) is the temperature atwhich the liquid crystal undergoes the nematic to isotropic transition,which is the transition from the orientationally ordered nematic phaseto the totally disordered isotropic phase. As used herein, T_(NI) refersto the nematic-isotropic transition temperature of the guest-hostmixture.

“Optical Device” refers to a device where the light transmission throughthe device can be controlled by application of a voltage. Such devicesinclude eyewear (such as sunglasses, ophthalmic glasses or lenses, eyeprotectors, visors, head mounted displays, etc), front layer ofauto-dimming mirrors, as well as windows and sunroofs, e.g. forbuildings, vehicle, airplanes, etc.

“Order parameter of the guest-host mixture” or “S_(mix)” refers to theorder parameter of the guest-host mixture. The mixture may contain oneor more dyes as well as other dopants. The S_(mix) can be measuredaccording to the method described herein, e.g. using the formula forEffective Order Parameter (S_(eff)) or Aggregate Effective OrderParameter (S_(eff-agg)). As used herein S_(mix), S_(eff) and S_(eff-agg)are used interchangeably (depending on which method is used to measurethe order parameter) and describe the same parameter.

“Passive layer” refers to a layer that does not alter its behavior inresponse to an applied voltage but may respond to other stimuli such astemperature, light, etc.

“Polarization dependence” is a measure of a material's response to twoorthogonal linear polarizations; i.e. where the optical properties of amaterial experienced by an incident light (such as index of refractionor absorption/transmittance) are dependent on the polarization of theincident light.

“Polarization sensitivity” is the relative measure of the response ofthe material between the two orthogonal linear polarizations. Zeropercent (0%) polarization sensitivity refers to a polarizationinsensitive device and a 100% polarization sensitivity refers to acompletely polarization sensitive device as obtained using a polarizer.

“Polarizer” refers to a material that absorbs or reflects onepolarization of incident light more than the orthogonal polarization.

“Transmission swing” refers to the difference in transmission betweenthe clear state and dark state transmissions. For example, if the clearstate transmission is 55% and the dark state transmission is 15%, thetransmission swing is 40%. The transmission swing of an optical devicecan be measured using equipment such as a “haze-gard plus” device fromBYK-Gardner, USA, or equivalent.

“Uniform optical retardation” refers to plastic substrates having anoptical retardation variation that is less than ±20. “Opticalretardation” is defined as the change in the optical phase experiencedby different polarizations of incident light.

“Wide band absorption” as used herein, is defined as a spectralabsorption band with an aggregate Full Width at Half Max (A-FWHM) thatis greater than 175 nm, and preferably greater than 180 nm, 185 nm, 190nm, 195 nm or 200 nm, where the entire spectral absorption band iscontained within the range of visible wavelengths, i.e. 400 nm-700 nm(See FIG. 2)

“Wide Band Device” refers to a device that exhibits a wide absorptionband, and a wide (i.e. >30%) transmission swing with polarizationsensitivity of less than 20%, or in some examples less than 15%, or insome examples less than 10%.

“Wide Band Mixture” refers to a guest-host liquid crystal mixture thatcan be used in a wide band device.

The Liquid Crystal Cell

FIGS. 1A and 1B generally show an example of an electronicallycontrolled liquid crystal cell capable of variably controlling lighttransmission. As seen in FIGS. 1A-B, a variable light transmissionguest-host cell according to the present invention is designatedgenerally by the numeral 10. The cell 10 includes two substrates, 12 a,12 b, with a substantially constant separation between them and enclosedon all sides by a sealing material 13, such as a UV cured opticaladhesive. As will be discussed in further detail below, a solution ofdichroic dye and a liquid crystalline material is disposed between thesubstrates 12 a and 12 b.

Substrates 12 a, 2 b are plastic, low-birefringence, light transmissivematerials, either the same or different. The inner surfaces of thesubstrates are coated with a transparent conducting layer, 14 a, 14 b,such as indium tin oxide (ITO) or conductive polymer. Both conductinglayers 14 a and 14 b are connected to a power circuit 15. The powercircuit 15 includes at least a variable voltage supply which isrepresented schematically in FIGS. 1A and 1B by the encircled V. Coatingthe inside of each conducting layer 14 may be an optional passivationlayer (also known as an insulating layer or “hard coat”), 16 a, 16 b,comprising, for example, a Si, Ti alcoxide. This layer is optional andin some examples, including the examples enumerated herein, it can beeliminated all together. The innermost layer is an alignment layer, 18a, 18 b, which can also act as a passivation layer.

Substrates 12 a, 12 b can be planar or curved. The distance betweensubstrates 12 a, 12 b has ramifications on properties of the cell. Ifthe host is a liquid crystal material, increasing the distance betweenthe substrates 12 a and 12 b tends to reduce the possibility of making apolarization insensitive device; decreasing the distance tends todecrease the light-absorption capacity of the cell and increase thedifficulty of manufacturing. This distance defines a cell thickness 20and in some examples is from 3 to 20 μm, or from 5 to 10 μm. To aid inmaintaining the separation, optional spacers 21, such as glass orplastic rods or beads, may be inserted between the substrates 12 a and12 b.

The guest-host solution of the present invention includes a guestdichroic dye 24 in a liquid crystal host material 22. Dichroic dye 24 isan organic molecule (or mixture of molecules) whose absorption ofpolarized light strongly depends on the direction of polarizationrelative to the absorption dipole in the molecule. Dichroic dye 24 haspositive dichroism in which a maximal absorption occurs when thepolarization is parallel to the long molecular axis of the dye moleculeand a minimal absorption occurs when the polarization is perpendicularto the long axis. The dichroic dye 24 is further described below.

Liquid crystals are inherently birefringent which can result inpolarization sensitivity of the device. Preferably, liquid crystalmaterial 22 is either nematic, chiral nematic or achiral nematicsupplemented with a chiral dopant. The liquid crystal material 22 mayinclude more chiral material if lower polarization sensitivity isdesired, or less chiral material if greater polarization sensitivity isdesired. However, the amount of chiral material is inversely related tothe pitch. Using a greater amount of chiral material results in ashorter pitch, and if the pitch is too short, it becomes difficult tocontrol the liquid crystal texture, which may result in formation offocal conic or finger print textures. Since these textures increasehaze, they can reduce performance for optical applications and should beavoided.

Cell 10 is either in a resting state, in which no voltage is applied, orin an energized state, in which a voltage is applied across twosubstrates. The present invention can be constructed so that theapplication of a voltage can either increase or decrease thetransmittance of light. In one embodiment the resting or de-energizedstate has the maximal light transmittance or “clear state”, as seen inFIG. 1A, and the active or energized state has minimal lighttransmittance or “dark state”, as seen in FIG. 1B. This can be achievedby use of a homeotropic surface treatment for alignment layers 18 a,b inconjunction with a dye having positive dichroism and a liquid crystalmaterial with negative dielectric anisotropy, as shown in FIGS. 1A and1B.

Accordingly, in this example, the liquid crystal (LC) and dye molecules202 have a first condition in the resting (no-voltage or de-energized)state in which the orientation of the dye molecule is vertical inrelation to the LC substrates (FIG. 1C), so the mixture is in a“clear-state” when no voltage is applied. When voltage is applied, theLC molecules have a second condition in which the orientation of the LCand dye molecules is at a more parallel angle in relation to the LCsubstrate (FIG. 1D), so the mixture absorbs light or is in a “darkstate”.

In another embodiment, the resting or de-energized state has the minimallight transmittance or “dark state” (LC orientation as shown in FIG.1D), and the active or energized state has maximal light transmittanceor “clear state” (as shown in FIG. 1C). This can be achieved by use of aplanar surface treatment for alignment layers 18 a,b in conjunction witha dye having positive dichroism and a liquid crystal material withpositive dielectric anisotropy.

The transition from one state to another may be abrupt (by switchingfrom one state to another with no intervening states) or graduated (e.g.by gradually increasing or decreasing tint and/or opacity, resulting inone or more intermediate states). Therefore, in some embodiments, thelevel of tint is variable depending on the voltage applied.

The voltage can be varied in a step-wise manner (using discrete steps),or gradually (using small increments, so as to impart a smoothtransition from one intermediate state to another), or a combinationthereof.

Furthermore, the different voltages may be applied automatically (e.g.according to the amount of light reaching the optical device as measuredby a light sensor), manually (e.g. according to user manipulation of acontrol circuit) or by a combination of automatic and manualapplication.

Characteristics of the Guest-Dye Mixture

To achieve the desirable characteristics for a wide band variabletransmission device, the dye mixture needs to have certaincharacteristics, described below.

Transmission Swing

A device according to the invention has a high contrast between clearand dark. The desired clear state transmission may be equal to or above30%, preferably equal to or above 35%, 40%, 45%, 50%, 55%, 60%, 65% or70%. The dark state transmission may be equal to or below 40%,preferably equal to or below 35%, 30%, 25%, 20%, 15% or 10%. Thetransmission swing between the clear and the dark state should be equalto or greater than 30%. Examples of transmission swing values include30%, 35%, 40%, 45%, 50%, 55% or 60%. Additionally, the transmission issubstantially uniform (±10%) across the surface of the cell. Suchhigh-transmission clear states permits one to keep wearing such eyewearin many low light situations, while such low-transmission dark statesprovide necessary vision protection.

Wide Band Absorption

Absorption as used herein is to define the percentage of light nottransmitted through the cell or device. It is related to transmission bythe following: absorption=100%−transmission. Absorption band is definedas the spectral wavelength wherein absorption occurs. “Wide bandabsorption” as used herein, is defined as a spectral absorption bandwidth that is greater than 175 nm in one embodiment, and greater than180 nm, 185 nm, 190 nm, 195 nm or 200 nm in other embodiments, where theentire spectral absorption band is contained within the range of visiblewavelengths, i.e. 400 nm-700 nm. A “wide band dye” is considered to beany dye or mixture of dyes that results in a wide absorption band asdefined herein.

The absorption band width, or the width of the absorption curve, iscalculated by what is known in the art as an “Aggregate Full Width atHalf Maximum” (A-FWHM), defined as the distance between the cut-offpoints on the absorption curve that occur where the absorption isone-half of the maximum absorption. See FIG. 2A. Thus, regardless of theshape of the absorption curve, the FWHM is the width of the curvebetween the two cut-off points where the absorption is one-half of themaximum absorption. Thus, “wide band” absorption as used herein refersto a dye, or cell, or device having an absorption curve with an A-FWHMthat is greater than 175 nm where the entire spectral absorption band iscontained within the range of visible wavelengths. In some embodiments,the A-FWHM is greater than 180 nm, 185 nm, 190 nm, 195 nm or 200 nm.

Since, for the present invention, the range of interest for absorptionis limited to visible wavelengths, a cut-off point can occur at 400 nmor 700 nm even if the absorption at those wavelengths is more thanone-half of the maximum absorption. See FIG. 2B. This definition of acut-off point implies that for example dyes or mixtures which result insignificantly different absorption curves could have the same FWHM (seeFIG. 2C). Thus, FWHM is based solely on the distance between the cut-offpoints, without regard to the detailed features of the absorption curve,e.g. if the curve has a long tail, this will not affect the FWHM. Dyeswith absorbance spectra that have two cut-off points are described ashaving a single peak, even if the detailed structure of the absorbancespectra between the cut-off points is complicated. (FIG. 2D) The presentinvention also includes dyes with absorbance spectra that have more thantwo cut-off points. Such dyes are described as having multiple peaks,where each peak is distinct and can be characterized by its own FWHM,which is measured in the same manner as described above for a singlepeak dye. An aggregate FWHM (A-FWHM) is defined as the sum of the FWHMof all peaks in the absorption spectrum. (See FIG. 2E). If there is onlyone peak, then an A-FWHM is simply the FWHM.

Order Parameter

The maximum contrast between the clear and dark states of an LC celldepends on the alignment of the dichroic dyes. Dichroic dyes have theability to align themselves with nematic liquid crystal molecules whenmixed together. When an electric field is applied to such a guest-hostmixture, the nematic liquid crystal host molecules reorient and aligneither with or perpendicular to the electric field in order to minimizethe torque they experience from the electric field. The dichroic dye(guest) molecules may not be directly affected by the external electricfield but can align themselves with the liquid crystal host molecules.It is their interaction with the liquid crystal molecules that forcesthem to reorient.

The statistically averaged orientation of the elongated molecules,liquid crystal and dichroic dye, in a guest-host mixture points in aparticular direction that is called the “director.” Since all moleculesin the mixture are subject to random thermal motion as they diffuse,each molecule will not point in exactly the same direction as thedirector, even when an electric field is applied. A statistical averageof the molecular orientation reveals that the molecules are tilted at anaverage angle θ_(avg) with respect to the director. This molecular tiltcan also be characterized and calculated by a useful quantity called the“order parameter, S”, which ranges in value from 0 to 1. An orderparameter of S=1 corresponds to all molecules being perfectly alignedwith the director (θ_(avg)=0°). (See Liquid Crystals Applications andUses, vol. 3, edited by B. Bahadur, published by World ScientificPublishing Co. Pte. Ltd., 1992). Thus, the higher the order parameter S,the more the dichroic dye molecules are aligned, thereby optimizingabsorption for any particular molecular orientation. (FIG. 3). Thepresent invention includes a dichroic dye liquid crystal guest-hostmixture with an effective order parameter S_(mix) which is greater thanor equal to 0.78, 0.79 or 0.8.

As used herein, the “guest-host mixture order parameter value” or“S_(mix)” refers to the order parameter of the guest-host mixture. Themixture may contain one or more dyes as well as other dopants. TheS_(mix) can be measured according to the method described herein, e.g.using the formula for S_(eff) or S_(eff-agg). Thus, as used hereinS_(mix), S_(eff) and S_(eff-agg) are used interchangeably (depending onwhich method is used to measure the order parameter) and describe thesame parameter. The “dye order parameter value” or “S_(dye)” refers tothe order parameter of the transition dipole of each dichroic dye withrespect to the director.

In one example, the effective order parameter of guest host mixturecontaining one or more dichroic dyes exhibiting a wide absorbancespectrum (e.g. a neutral dye) is calculated as S_(eff)=(D_(eff)−1)(D_(eff)+2), where D_(eff)=(∫A_(∥)(λ)dλ)/(∫A_(⊥)(λ)dλ) is the “effectivedichroic ratio” and A_(∥)(λ) and A_(⊥)(λ) are the parallel andperpendicular absorbance of the dye at wavelength λ. Typically,∫A_(∥)(λ)dλ and ∫A_(⊥)(λ)dλ are evaluated over the 380-780 nm region ofthe spectrum. For the present invention, these integrals are evaluatedover the FWHM of the absorption spectrum of the wide band dichroic dyemixture, which, as previously described, is limited to the 400-700 nmregion of the spectrum. If the absorption spectrum has a single peak,the integrals are simple to evaluate, the integration limits being thewavelengths of the end-points of the FWHM of the spectrum. If there ismore than one distinct peak in the absorption spectrum, the integralsare evaluated in a piece-wise fashion, the integration limits being thewavelengths of the end-points of the FWHM of each peak. This piece-wiseintegration produces what the Applicant calls an “aggregate dichroicratio” D_(eff-agg) and an “aggregate effective order parameter”S_(eff-agg).

The order parameter of the mixture can be determined by opticalmeasurements of the light transmission in the resting and energizedstates using linearly and/or circularly polarized lights at severalwavelengths both within and outside of the absorption spectrum. Then,using liquid crystal optics simulation methods such as those developedby Berreman, (Berreman D. W. 1972, Optics in Stratified and AnisotropicMedia: 4×4-Matrix Formulation. Journal of the Optical Society ofAmerica, 62(4), 502). or Odano (Allia, P., Oldano, G., & Trossi, L.,1986, 4×4 Matrix approach to chyral liquid-crystal optics. Journal ofthe Optical Society of America B, 3(3), 424); the order parameter can bedetermined by numerical fitting to the experimental data. Thesesimulation methods are used by those skilled in the art or throughcommercial programs such as Twisted Cell Optics by Kelly (Kelly, J.,Jamal, S., & Cui, M., 1999, Simulation of the dynamics of twistednematic devices including flow. Journal of Applied Physics, 86(8),4091).

For the purposes of this invention, an order parameter of 1 alsoindicates that all the molecules are aligned with each other. Forexample, all the dichroic dye molecules are aligned with each other,presenting near identical absorption cross-sections to the incidentlight and maximizing absorption for that particular orientation. Ofcourse, it must be kept in mind that perfect alignment is difficult toachieve since the molecules are always subject to thermal motion. Tomaximize optical performance, a guest-host mixture is desired whereinthe inter-molecular alignment is increased because of the application ofan external field.

In some examples, a desirable guest-host mixture will have an orderparameter value S_(mix) of greater than 0.78. In other examples, theS_(mix) is equal to or greater than 0.79. In yet other examples, the isequal to or greater than 0.8. Mixtures with S_(mix)>0.78 are needed thatprovide a wide transmission swing (30-70%, preferably >35%) across theA-FWHM.

In some examples, if more than one dye is used, to minimize colorvariation in the resting (de-energized) and energized states, it isdesirable that all dyes have approximately the same order parameter.

“Dichroic ratio”, “average dichroic ratio” or D_(mix) of the mixture,similarly, refers to the dichroic ratio of the guest-host mixture whichmay contain one or more dichroic dyes. As explained above, the dichroicratio may be measured using the formula for D_(eff) or D_(eff-agg).Thus, as used herein, D_(mix), D_(eff) or D_(eff-agg) are usedinterchangeably (depending on which method is used to measure thedichroic ratio) and describe the same parameter.

In some examples, a desirable guest-host mixture will have a dichroicratio D_(mix) greater than 11.5 at wavelengths within the A-FWHM. Inother examples, the dichroic ratio D_(mix) is greater than 12, orgreater than 12.5, or greater than 13 at wavelengths within the A-FWHM.

The guest-host mixture includes a chiral liquid crystal host. Thechirality of the host material results in an intrinsic pitch, p, of theliquid crystal host material. This parameter is relevant to theperformance. In addition, the ratio of the cell gap thickness, d, tothis pitch is referred to as d/p. The pitch is a function of the helicaltwisting power and the concentration of the dopant, and can bedetermined as known in the art. For the present application the ratiod/p is dependent on whether the cell is clear in the Off state (wherethe liquid crystal host of the guest-host mixture has a negativedielectric anisotropy, referred to herein as a “negative mixture”), orwhether the cell is dark in the off state (where the liquid crystal hostof the guest-host mixture has a positive dielectric anisotropy, referredto herein as a “positive mixture”).

Thus, where the mixture is a negative mixture, the d/p is selected to beless than 1 but greater than or equal to 0.25. In some examples, the d/pis less than 0.9, or in some examples, less than 0.8 or less than 0.7.

Alternatively, where the mixture is a positive mixture, the d/p isselected to be less than 5 but greater than or equal to 1.75. In someexamples, the d/p is less than 4, or in some examples, less than orequal to 3.

In general, in certain examples, the guest-host mixture is polymer freeor substantially polymer free to reduce haze.

Solubility

A dichroic dye must also be sufficiently soluble in the liquid crystalhost so that an appropriate concentration of dye can be reached toproduce the desired absorption in the dark state. Increasing the lengthof a dye (e.g. more azo groups) creates a limit for the concentration ofthe dye within the liquid crystal host. This limits the total absorbingpower of the LC-dye mixture, hence affecting the transmission window.The trade-offs made when selecting a dichroic dye and formulating aguest-host mixture complicate the development process for variabletransmission films. We have found that structural similarities of thedye with at least one or more components of the liquid crystal host, insome instances, allow us to circumvent this limitation and increasesolubility. For example, we have found that some modifications of a dyetail may not affect the overall performance of the dye but can allowadditional dyes to be dissolved in the mixture. In effect, we canincrease the total amount of dissolved dye by carefully matching the dyewith the host. In some examples, the mixture contains 1-10% dye of orderparameter of >0.78 in the liquid crystal mixture.

Tail groups include moieties that are structurally compatible with theformation of liquid crystal phases and contain at least one ring orpreferably two rings or more, connected to each other through a covalentbond or a linking unit. The rings, which may be the same or different,may include 5- or 6-membered aromatic or non-aromatic rings. The ringsmay be selected independently from benzene, substituted benzene,naphthalene, substituted naphthalene, cyclohexane, substitutedcyclohexane, heterocyclic rings and substituted heterocyclic rings.Examples of heterocyclic rings include 5- or 6-membered rings and mayinclude one or more members selected from nitrogen, oxygen, and sulfur.

Examples of tail groups include groups that may be represented by thefollowing formulas:

wherein each R¹ and R² is selected independently from the groupconsisting of hydrogen, halogen, —R^(a), —OH, —OR^(a), —O—COR^(a), —SH,—SR^(a), —NH₂, —NHR^(a), —NR^(a)R^(a), —NR^(b)R^(c); wherein R^(a) is alinear or branched (C₁₋₁₈)alkyl group, a linear or branched(C₁₋₁₈)alkenyl group or a linear or branched (C₁₋₁₈)haloalkyl group;R^(b) and R^(c) are independently selected from the group consisting ofhydrogen and linear or branched (C₁₋₁₈)alkyl groups; or wherein R^(b)and R^(c) combine to form a saturated 5- to 7-member heterocyclic group.In these formulas, n is an integer from 1 to 5; m is an integer from 0to 4; X¹, X², and X³, identical or different from each other, arecovalent bonds or linking units; and Y is oxygen, nitrogen, or sulfur.Linking units include divalent organic groups. Examples of linking unitsinclude alkyl, ether, ester, ethylene, acetylene, imino, azo, and thiogroups. Linking units include groups that may be represented by theformulas —R^(d)—, —O—, —OR^(d)—, —OR^(d)O—, —OCO—, —OCOR^(d),—OCOR^(d)O—, —S—, —CH═CH—, —CH═N—, —C≡C—, wherein R^(d) is a linear orbranched (C₁₋₁₈)alkyl group or a linear or branched (C₁₋₁₈) haloalkylgroup.Nematic-Isotropic Temperature (T_(NI))

Another important parameter is the temperature at which the liquidcrystal undergoes the nematic to isotropic transition, T_(NI), which isthe transition from the orientationally ordered nematic phase to thetotally disordered isotropic phase. In the disordered state, applying avoltage will have no effect on light transmission. We have found thatfor the present invention the liquid crystal mixture combined with thedye should have a T_(NI) of greater than about 40° C. in one embodiment,and greater than about 65° C. in another embodiment. In some examples,the T_(NI) is greater than 45° C., 50° C., 55° C., 60° C., 65° C., 70°C., 75° C., 80%, 85% or 90° C.

Adding the dye to the liquid crystal host changes the T_(NI) of themixture. Therefore the selection of the mixture must be made takingT_(NI) into consideration.

Dyes

We have determined that a class of dyes which can satisfy the aboveconditions is azo based dyes. More particularly, we have found that dyeswhich have at least two azo groups are desired. The azo dye may have anextended core, e.g. 2-10 azo groups. In some examples, the dye has 2-6azo groups. Of this group, dyes should be used that are compatible withthe liquid crystalline host that will be used. People of ordinary skillin the art appreciate that the absorption band and the order parameterof the dye can be altered through proper selection of core andsubstituent. Some examples of the dyes that can be used in a mixture areshown in the table below. (See Liquid Crystals Applications and Uses,vol. 3, edited by B. Bahadur, published by World Scientific PublishingCo. Pte. Ltd., 1992, p. 73-81). For example, these dyes are sufficientlysoluble in the liquid crystal hosts and the order parameter of theseguest-host mixtures is equal or greater than 0.8.

Order param- Dye Structure eter LC Host G-207

0.826 ZLI1840 G-241

0.820 MLC6609 2.27

0.800 EK22650Plastic Substrates

The present inventors discovered that high-performance wide banddichroic dyes reveal optical flaws in the plastic substrates that hadpreviously been used successfully by the Applicant with narrow band(<175 nm) dichroic dyes.

Unlike other liquid crystal based applications, the present inventionrequires the use of plastic substrates each with a thickness less than:750, 500 or 250 micrometers. These can then be laminated to thickercarriers to provide mechanical stability. The substrates can be composedof thermoplastic materials, such as PET, PES, TAC, Polycarbonate, orsimilar materials, or thermoset materials such as CR39, or otherthermoset materials known in the art. It should be noted that eachplastic has unique optical properties. Our investigations showed thatplastics generally exhibit optical retardation. “Optical retardation” isdefined as the change in the optical phase experienced by differentpolarizations of incident light. As understood by people experienced inoptics, optical phase depends on the refractive index, thickness of thecell, and wavelength of the incident light for each polarization. It hasbeen assumed in the art that for optical devices based on liquidcrystals the optical retardation of the substrate must be the same asthe optical retardation of glass, which is zero. This has put asubstantial limitation on the development of plastic eyewear devices andmany plastic substrates which have an inherent retardation have beendeemed unacceptable. The basis for this assumption is that since anyliquid crystal device has some polarization dependence, any retardationin the substrate will be observable, and as such, only substrates withno birefringence can be used. This can be seen in the use ofbirefringence free plastics such as TAC for optical polarizers.

However, we have discovered that it is not the actual retardation butthe relative retardation that is more important. In particular, if theplastic is designed to have retardation with less than ±20% variation inuniformity across the area of the device it is possible to avoid theabove limitation and achieve wide band absorption. Therefore, we havediscovered that the actual retardation value limitation can becircumvented if we choose a plastic whose retardation value is moreuniform across the device (i.e. less than 20% variation). This is asubstantial departure from conventional thinking about plastics andopens the potential for wide absorption band devices. In particular, itis found that a variation of less than ±20%, or in some examples, avariation of less than ±15% or less than ±10% is desirable. This can bemost readily achieved if the plastic has overall minimal retardation,such as TAC, or if it is made to have a more consistent (+/−20%)retardation across the substrates. This can be achieved by properstretching of plastics which have traditionally been discounted assubstrates for liquid crystal optical applications including PET, PES orPC. In addition, we have discovered that the stretching mode can have asubstantial effect on the performance. For optimal performance,stretching must be such that the angular deviation between the twoorthogonal modes of the plastic is less than 30 degrees within the areaof the optical device used. It is well known in the art that suchdeviations can be achieved by proper stretching methods.

Electronically Passive Layer

An electronically passive layer is a layer that further reduces thetransmission of light through the device by either absorbing, reflectingor scattering a portion of incident light, or a combination thereof. Asthe name suggests this layer is not activated by a voltage, e.g. it isnot a liquid crystal or electrochromic layer. In some examples, thislayer is polarization independent (wherein it absorbs or reflects orscatters incident light regardless of its polarization). In otherexamples, this layer is polarization dependent, i.e. it absorbs aselected polarization of light more than another polarization (such asan absorptive polarizer); or it reflects one polarization of light morethan another polarization (such as a reflective polarizer). Anabsorptive polarizer typically has two axes, an absorptive axis and atransmissive axis, which are at right angles to each other. Thepolarization of the light that is parallel to the absorptive axis isabsorbed more than the polarization parallel to the transmissive axis.Similarly, a reflective polarizer has two axes, a reflective axis and atransmissive axis, such that the polarization of light that is parallelto the reflective axis is reflected more than the polarization parallelto the transmissive axis. Examples of absorptive and reflectivepolarizers are well known in the art.

An electronically passive layer may be activated by parameters otherthan voltage, for example by temperature or by photons (in case of alayer containing photochromic or photochromic-dichroic dyes). Anelectronically passive layer may also combine one or more absorptive,reflective or scattering layers, and may combine different polarizationindependent and polarization dependent layers, according to need andapplication.

In one application, a window or sunroof of a vehicle or aircraftincludes an optical device that includes an electronically active liquidcrystal cell laminated or otherwise attached to a glass or plasticwindow panel, where the window panel acts as an electronically passivelayer which can be an absorptive and/or reflective and/or scatteringlayer (or any combination of these), and can be either polarizationdependent or independent. In such an example, it may be desirable tohave an arrangement where the optical device has a dark state (leasttransmission) when there is no voltage applied to the cell, and where itcan increase the transmission of light upon application of a voltage, upto a “clear” or most transmissive state when a maximum voltage isapplied. This arrangement employs a liquid crystal host with positivedielectric anisotropy. Example 3 below describes a cell having such anarrangement.

Second Electronically Active Layer (Double Stack)

In another aspect of the present invention, the optical device maycontain a second electronically active cell that contains a liquidcrystal—dye mixture for further reducing the transmission of lightthrough the optical device. This second electronically active cell willabsorb or scatter a portion of light in either the Off state, when novoltage is applied (e.g. when the cell has an LC host with positivedielectric anisotropy), or in the On state, in response to an appliedvoltage (i.e. when the cell has an LC host with negative dielectricanisotropy).

Several versions of this optical device are contemplated. For example,in one version, the first electronically active cell itself exhibits awide absorption band that is greater than 175 nm within a visiblewavelength range of 400-700 nm. In another version, the first and thesecond electronically active cells each exhibit a wide (greater than 175nm) absorption band within the wavelength range of 400-700 nm. In yetanother version, the first and the second electronically active cells incombination exhibit a wide (greater than 175 nm) absorption band withinthe wavelength range of 400-700 nm.

Similarly, in one version, the first electronically active cell itselfexhibits a clear state transmission equal to or above 30% and a darkstate transmission equal to or below 40%. In other embodiments, thecombination of the first and second electronically active cells exhibita clear state transmission equal to or above 30% and a dark statetransmission equal to or below 40%. In some embodiments, the clear statetransmission may be equal to or above 35%, 40%, 45%, 50%, 55%, 60%, 65%or 70%. In some embodiments, the dark state transmission may be equal toor below 35%, 30%, 25%, 20%, 15%, 10% or 5%.

In some embodiments, the parameters of the second electronically activecell such as the guest-host mixture, order parameter, d/p, T_(NI),substrate characteristics, absorption band width, clear state and darkstate transmissions, and transmission swing may be similar or identicalto the first cell. In other embodiments, the second cell may havecompletely different parameters, which may be chosen in accordance witheach application the device is used for.

Cell with Scattering Capability

In some embodiments, the electronically active guest-host cell itselffurther includes an ionic element to induce dynamic scattering or flowof the liquid crystal molecules in the cell, thereby causing scatteringof light or “haze”. This scattering state is shown in FIG. 1E and isreferred to as “opaque” since it results in an opaque or “frosted”appearance.

A dynamic scattering state can control the amount of light that can passthrough a device by inducing (via an electric field) a stronglyturbulent fluid flow in the liquid crystal. Since the host liquidcrystal's orientation direction (and thus its optic axis) is coupled tothe fluid flow, the turbulence creates sustained and large temporal andspatial variations in the optic axis; these strongly scatter light sothat very little light that enters the liquid crystal can exit with itsdirection and polarization state unchanged. When the electric field isremoved, the liquid crystal reverts to its transparent (clear) state.These systems create a hazy film that precludes visual images from theviewer and are used wheree optical clarity is not needed.

Dyes can be added to the system, in order to introduce color or toenhance the scattering effect. In this event, the large variations inthe liquid crystal optic axis are able to strongly scatter light.Because of the presence of the dichroic dye guest in the cell, and sincethe optic axis of the liquid crystal is no longer parallel to thelight's propagation direction, the axis of the dichroic dye molecules isfrequently perpendicular to the light's local propagation direction, andso much light is absorbed by the dye. In this event, the device isOPAQUE: that is, it contains minimal transparency because it ismaximally tinted (has high-tint due to absorption of light by the dyes)and is maximally hazy (has high-haze due to the dynamic scatteringeffect).

The choice between the class of material and method of light controldepends on the particular application. Some applications require both ascatter free change in the transmission and/or a scatter based change intransmission. This optical device can be used in such applications.

In some embodiments, the device is configured so that in the absence ofa voltage, the guest-host mixture molecules are orientated in a maximumtransparency direction to achieve a low-tint (and low-haze) Clear state.Upon application of a first voltage, the mixture molecules undergodynamic scattering to achieve a high-tint and high-haze “Opaque” state.

In another embodiment, the device is configured so that upon applicationof a second voltage, the mixture molecules orient to absorb light withminimal scattering, thereby achieving a high-tint but low haze(“Tinted”) state.

In some examples, the device has a different mode of operation in thatthe liquid crystal material in the device has a positive dielectricanisotropy, so that in the absence of a voltage or when the firstvoltage has an amplitude below a predetermined voltage threshold value,the device is in the opaque or dark state, and upon application of avoltage, the device can become clear. In some examples, upon applicationof a second voltage, the device can become tinted whereby the mixturemolecules orient to absorb light with minimal scattering, therebyachieving a high-tint but low haze.

In some applications, the device can have one or more regions, eachregion having a liquid crystal cell according to the invention,configured so that each region can be operated independently and cantransition from one state to another independently of the other regions.Thus, for example, a first portion of the device can be in the clear ortinted state to allow ambient light to reach a viewer, while independentof the first portion, a second portion of the device can be in an opaquestate, to allow, for example, a displayed image to be seen by theviewer, or, to provide a privacy window.

EXAMPLE 1

A variable transmission cell was prepared according to the followingprotocol. A test cell was fabricated using substrates of 5 milpolyethylene terephthalate (PET) pre-coated with a conducting polymer(Kimoto Tech, Cedartown Ga., U.S.A.). The conductive polymer served as atransparent electrode. On top of the conducting polymer, a coating ofpolyimide, Nissan SE1211 (Nissan Chemical Industries, Ltd., Tokyo,Japan), was applied by silk-screening and then baked at 100° C. for 2hours. This polyimide coating served as an alignment layer designed toinduce a substantially homeotropic surface alignment of the liquidcrystal molecules. Next, Shinshikyu EW plastic spheres, 6 micron indiameter, (Hiko Industrial Ltd, Hong Kong) were sprayed onto one of thesubstrates to act as spacers. A thin bead of UV curable adhesive,Loctite 3106, (Henkel AG & Co. KGaA, Dusseldorf, Germany) was thenapplied around the perimeter of one of the substrates, leaving a gapthat would serve as a fill port. The two substrates were then assembled,pressed together against the spacers to create a uniform gap between thesubstrates, and then exposed to UV light to cure the adhesive.

A guest-host mixture was then prepared that consisted of (1) 94.8% byweight of negative dielectric anisotropy liquid crystal host, MLC-6609,from Merck (EMD Chemicals, Gibbstown, N.J., U.S.A.); which has anegative dielectric anisotropy (Δ∈<0); (2) 1.125% chiral dopant, ZLI811,also from Merck; and (3) a azo based dichoric dye mixture consisting of0.41% of dye DR-1303; (AlphaMicron, USA), 0.95% of G-241; (MarubeniChemicals, Japan), and 2.71% total of dyes LSY-210; (Mitsubishi ChemicalCorporation, Japan), DD-1123, DD1032, DD1089; (AlphaMicron Inc, USA)mixed in equal ratios. The test cell was placed in a vacuum chamber toremove air in the gap between the substrates and then filled with theguest-host mixture by capillary action. The fill port was sealed usingthe UV curable adhesive. A conductive tape, consisting of a copperbacking and conducting adhesive, was then adhered to the conductingpolymer coating on each substrate to serve as robust interconnects forelectrical leads.

The absorption curve of the cell is shown in FIG. 4. The dichroic ratioand order parameter of the mixture were measured and calculated asdescribed herein. The cell had a photopic transmission swing>30%, and aneffective dichroic ratio of 14 (order parameter, S_(mix)=0.82), a d/pratio of 0.75, and a T_(NI) of 91.5° C. (LC host).

EXAMPLE 2

A variable transmission cell was prepared according to the followingprotocol. A test cell was fabricated using substrates of 3 milpolyethylene terephthalate (PET) pre-coated with a transparentconducting ITO (Techimat, U.S.A.). On top of the conducting polymer, acoating of polyimide, Nissan SE1211 (Nissan Chemical Industries, Ltd.,Tokyo, Japan), was applied by silk-screening and then baked at 100° C.for 2 hours. This polyimide coating served as an alignment layerdesigned to induce a substantially homeotropic surface alignment of theliquid crystal molecules. Next, Shinshikyu EW plastic spheres, 6 micronin diameter, (Hiko Industrial Ltd, Hong Kong) were sprayed onto one ofthe substrates to act as spacers. A thin bead of UV curable adhesive,Loctite 3106, (Henkel AG & Co. KGaA, Dusseldorf, Germany) was thenapplied around the perimeter of one of the substrates, leaving a gapthat would serve as a fill port. The two substrates were then assembled,pressed together against the spacers to create a uniform gap between thesubstrates, and then exposed to UV light to cure the adhesive.

A guest-host mixture was then prepared that consisted of: (1) 95.2% byweight of negative dielectric anisotropy liquid crystal host, MLC-6609,from Merck (EMD Chemicals, Gibbstown, N.J., U.S.A.); which has anegative dielectric anisotropy (Δ∈<0); (2) 0.9% chiral dopant, ZLI811,also from Merck; and (3) a azo based dichoric dye mixture consisting of0.38% of dye DR-1303; (AlphaMicron, USA), 0.76% of dye G-241; (MarubeniChemicals, Japan), and aggregate 1.51% of dyes LSY-210; (MitsubishiChemical Corporation, Japan), DD-1123, DD-1032, DD-1089; (AlphaMicronInc, USA) in equal amounts and aggregate 1.2% of DD-1112, DD-1215(AphaMicron USA) in a 2:1 ratio respectively. The test cell was placedin a vacuum chamber to remove air in the gap between the substrates andthen filled with the guest-host mixture by capillary action. The fillport was sealed using the UV curable adhesive. A conductive tape,consisting of a copper backing and conducting adhesive, was then adheredto the conducting polymer coating on each substrate to serve as robustinterconnects for electrical leads.

The absorption curve of the cell is shown in FIG. 5. The dichroic ratioand order parameter of the mixture were measured and calculated asdescribed herein. The cell had a photopic transmission swing of 40%, andan effective dichroic ratio of 15 (order parameter, S_(mix)=0.83), a d/pratio of 0.5, and a T_(NI) of 93° C. (LC host).

EXAMPLE 3

A variable transmission cell having a positive mixture was preparedaccording to the following protocol. A test cell was fabricated usingsubstrates of 3 mil polyethylene terephthalate (PET) coated with IndiumThin Oxide (ITO), a transparent conductor. On top of the ITO, a coatingof polyimide, Nissan 5291 (Nissan Chemical Industries, Ltd., Tokyo,Japan), was spin coated. This polyimide coating served as an alignmentlayer designed to induce a strong planar alignment of the liquid crystalmolecules. Next, L-34N black plastic spheres, 9.4 microns in diameter,(Hayakawa Rubber Co., Ltd, Hiroshima Japan) were sprayed onto one of thesubstrates to act as spacers. A thin bead of UV-curable adhesive,Loctite 3106, (Henkel AG & Co. KGaA, Dusseldorf, Germany) was thenapplied around the perimeter of one of the substrates, leaving a gapthat would serve as a fill port. The two substrates were then assembled,pressed together against the spacers to create a uniform gap between thesubstrates, and then exposed to UV light to cure the adhesive.

A guest-host mixture was prepared that consisted of: (1) 91.68% byweight of positive dielectric anisotropy liquid crystal host,HTW109100-100, from HCCH (Jiangsu Hecheng Display Technology CO., LTD,Jiangsu, China); which has a positive dielectric anisotropy (Δ∈>0); (2)2.76% of chiral dopant, ZLI811, from Merck (EMD Chemicals, Gibbstown,N.J., U.S.A); and (3) dichroic dye mixture consisting of 0.52% of dyeDR-1303 (AlphaMicron Inc., USA), 1.74% of blue dye (LSB-541; MitsubishiChemical Corporation, Japan), 1.1% of purple dye (G-241; MarubeniChemicals, Japan), and 2.2% total of DD 1123, DD1089, DD1032(AlphaMicron Inc., USA) mixed in equal ratios. The test cell was placedin a vacuum chamber to remove air in the gap between the substrates andthen filled with the guest-host mixture by capillary action. The fillport was sealed using the UV curable adhesive. A conductive tape,consisting of a copper backing and conductive adhesive, was then adheredto the ITO on each substrate to serve as robust interconnects forelectrical leads.

The absorption curve of the cell is shown in FIG. 6A. The dichroic ratioand order parameter of the mixture were measured and calculated asdescribed herein. The cell had a photopic transmission swing>30% (15V at200 Hz), an effective dichroic ratio of 13.6 (order parameter,Smix=0.808), a d/p ratio of 3.0, and a T_(NI) of 104.1° degrees (LChost). FIG. 6B shows the transmittance spectra of the cell.

EXAMPLE 4 Double Stack

A variable transmission double cell, which may be curved, is shown inFIG. 8 and was prepared according to the following protocol. Each testcell was fabricated using isotropic substrates of 3 mil polycarbonate(PC) coated with Indium Thin Oxide (ITO), a transparent conductor. Ontop of the ITO, a coating of polyimide, Nissan 5661 (Nissan ChemicalIndustries, Ltd., Tokyo, Japan), was spin coated. This polyimide coatingserved as an alignment layer designed to induce a strong homeotropicalignment of the liquid crystal molecules. Next, L-34S black plasticspheres, 6.2 microns in diameter, (Hayakawa Rubber Co., Ltd, HiroshimaJapan) were sprayed onto one of the substrates to act as spacers. A thinbead of UV-curable adhesive, Loctite 3106, (Henkel AG & Co. KGaA,Dusseldorf, Germany) was then applied around the perimeter of one of thesubstrates, leaving a gap that would serve as a fill port. The twosubstrates were then assembled, pressed together against the spacers tocreate a uniform gap between the substrates, and then exposed to UVlight to cure the adhesive.

For Cell 1, a guest-host mixture was prepared that consisted of (1)97.53% by weight of negative dielectric anisotropy liquid crystal host,MLC-6609, from Merck (EMD Chemicals, Gibbstown, N.J.); which has anegative dielectric anisotropy (Δ∈<0); (2) 0.8% of chiral dopant, L-811,also from Merck (EMD Chemicals, Gibbstown, N.J.), U.S.A; and (3)dichroic dye mixture consisting of 0.05% of dye DR-1303 (AlphaMicronInc., USA), 0.43% of blue dye (LSB-541; Mitsubishi Chemical Corporation,Japan), 0.3% of purple dye (G-241; Marubeni Chemicals, Japan), and 0.89%total of DD 1123, DD1089, DD1032 (AlphaMicron Inc., USA) mixed in equalratios.

For Cell 2, a guest-host mixture was prepared that consisted of: (1)97.53% by weight of negative dielectric anisotropy liquid crystal host,MLC-6609, from Merck (EMD Chemicals, Gibbstown, N.J.); which has anegative dielectric anisotropy (Δ∈<0); (2) 0.8% of chiral dopant, R-811,also from Merck (EMD Chemicals, Gibbstown, N.J.), U.S.A; and (3)dichroic dye mixture consisting of 0.05% of dye DR-1303 (AlphaMicronInc., USA), 0.43% of blue dye (LSB-541; Mitsubishi Chemical Corporation,Japan), 0.3% of purple dye (G-241; Marubeni Chemicals, Japan), and 0.89%total of DD1123, DD1089, DD1032 (AlphaMicron Inc., USA) mixed in equalratios.

In some embodiments, an optional support layer may be provided betweenthe two cells. In other embodiments, the support layer may be disposedon either side of the double cell. The support layer provides addedmechanical strength to the cell without adversely affecting the cell'soptical properties. The support layer may be any polymeric material.

Each test cell was placed in a vacuum chamber to remove air in the gapbetween the substrates and then filled with the particular guest-hostmixture by capillary action. The fill port was sealed using the UVcurable adhesive. A conductive tape, consisting of a copper backing andconductive adhesive, was then adhered to the ITO on each substrate toserve as robust interconnects for electrical leads. Cell 1 and Cell 2were laminated together with Optically Clear Adhesive 8213CL (3M, USA)

The absorption curve of the double cell system is shown in FIG. 7A. Thedichroic ratio and order parameter of the mixture were measured andcalculated as described herein. The cell had a photopic transmissionswing>55% (8V at 60 Hz), and an effective dichroic ratio of 16.5 (orderparameter, Smix=0.838), cell 1 has a d/p ratio of 0.5 (left handed),cell 2 has a d/p ratio of 0.5 (right handed), and a T_(NI) of 120° (LChost). FIG. 7B shows the transmittance spectra of the double cell. Ascan be seen from these figures, the absorption band width of thismixture is greater than 300 nm (spanning the entire spectrum 400-700nm).

The invention claimed is:
 1. An optical device comprising: anelectronically active cell comprising a guest-host mixture of a liquidcrystal host and a guest dye material comprising one or more dichroicdyes, contained between a pair of plastic substrates, wherein theguest-host mixture has an axis orientation that is alterable between aclear state orientation and a dark state orientation perpendicularthereto; wherein said guest-host mixture has an order parameter,S_(mix), greater than 0.78 and a nematic-isotropic transitiontemperature T_(NI) greater than 40° C.; wherein said pair of plasticsubstrates have an optical retardation with less than ±20% variation inuniformity for any given wavelength of incident light; wherein saidoptical device further comprises an electronically passive layer forfurther reducing the transmission of light through said optical device;wherein said optical device exhibits a wide absorption band having anaggregate full width at half maximum (A-FWHF) that is greater than 175nm within a visible wavelength range of 400-700 nm; and wherein saidoptical device has a variable transmission with a dark statetransmission equal to or below 40% and a transmission swing between theclear state orientation and the dark state orientation greater than orequal to 30%.
 2. The optical device of claim 1, wherein said guest-hostmixture has a clear state orientation when no voltage is applied to thecell and a dark state orientation when a voltage is applied to the cell,and wherein said liquid crystal host comprises a chiral nematic materialwith a thickness to pitch ratio (d/p) of less than 0.9 but greater thanor equal to 0.25.
 3. The optical device of claim 1, wherein saidguest-host mixture has a dark state orientation when no voltage isapplied to the cell and a clear state orientation when a voltage isapplied to the cell and wherein said liquid crystal host comprises achiral nematic material with a thickness to pitch ratio (d/p) of lessthan 5 but greater than or equal to 1.75.
 4. The optical device of claim1, wherein said electronically passive layer is a polarizationindependent absorptive, reflective or scattering layer, or a combinationthereof.
 5. The optical device of claim 4, wherein said electronicallypassive layer comprises a photochromic dye.
 6. The optical device ofclaim 1, wherein said electronically passive layer is a polarizationdependent absorptive or reflective layer, or a combination thereof. 7.The optical device of claim 1, wherein said one or more dichroic dyes inthe guest dye material are azo-based dyes having at least two azogroups.
 8. The optical device of claim 1, wherein said electronicallyactive cell itself exhibits a wide absorption band having an aggregatefull width at half maximum (A-FWHF) that is greater than 175 nm within avisible wavelength range of 400-700 nm, a variable transmission with adark state transmission equal to or below 40% and a transmission swingbetween the clear state orientation and the dark state orientationgreater than or equal to 30%.
 9. The optical device of claim 1, whereinsaid optical device is curved in at least one dimension.
 10. The opticaldevice of claim 1, wherein said optical device is a window or sunroof ofa vehicle.
 11. An optical device comprising: a first electronicallyactive cell comprising a first guest-host mixture of a liquid crystalhost and a guest dye material comprising one or more dichroic dyes,contained between a first pair of plastic substrates, wherein the firstguest-host mixture has an axis orientation that is alterable between aclear state orientation and a dark state orientation perpendicularthereto; wherein said first guest-host mixture has an order parameter,S_(mix), greater than 0.78 and a nematic-isotropic transitiontemperature T_(NI) greater than 40° C.; wherein said first pair ofplastic substrates have an optical retardation with less than ±20%variation in uniformity for any given wavelength of incident light;wherein the optical device further comprises a second electronicallyactive cell comprising a liquid crystal—dye mixture for further reducingthe transmission of light through said optical device; wherein saidoptical device exhibits a wide absorption band having an aggregate fullwidth at half maximum (A-FWHF) that is greater than 175 nm within avisible wavelength range of 400-700 nm; and wherein said optical devicehas a dark state transmission equal to or below 40% and a transmissionswing between the clear state orientation and the dark state orientationgreater than or equal to 30%.
 12. The optical device of claim 11,wherein said first guest-host mixture has a clear state orientation whenno voltage is applied to the cell and a dark state orientationperpendicular thereto when a voltage is applied to the cell, and whereinsaid liquid crystal host comprises a chiral nematic material with athickness to pitch ratio (d/p) of less than 0.9 but greater than orequal to 0.25.
 13. The optical device of claim 11, wherein said firstguest-host mixture has a dark state orientation when no voltage isapplied to the cell and a clear state orientation perpendicular theretowhen a voltage is applied to the cell and wherein the liquid crystalhost comprises a chiral nematic material with a thickness to pitch ratio(d/p) of less than 5 but greater than or equal to 1.75.
 14. The opticaldevice of claim 11, wherein said second electronically active cellabsorbs or scatters a portion of light in response to an appliedvoltage.
 15. The optical device of claim 11, wherein said secondelectronically active cell is absorptive or scattering in the absence ofan applied voltage.
 16. The optical device of claim 11, wherein saidfirst electronically active cell itself exhibits a wide absorption bandhaving an aggregate full width at half maximum (A-FWHF) that is greaterthan 175 nm within a visible wavelength range of 400-700 nm, a variabletransmission with a dark state transmission equal to or below 40% and atransmission swing between the clear state orientation and the darkstate orientation greater than or equal to 30%.
 17. The optical deviceof claim 11, wherein said first electronically active cell and saidsecond electronically active cell each exhibit a wide absorption bandhaving an aggregate full width at half maximum (A-FWHF) that is greaterthan 175 nm within a visible wavelength range of 400-700 n.
 18. Anoptical device comprising: an electronically active cell comprising aguest-host mixture of a liquid crystal host and a guest dye materialcomprising one or more dichroic dyes, contained between a pair ofplastic substrates, wherein the guest-host mixture has an axisorientation that is alterable between a clear state orientation and adark state orientation perpendicular thereto; wherein said guest-hostmixture has an order parameter, S_(mix), greater than 0.78 and anematic-isotropic transition temperature T_(NI) greater than 40° C.;wherein said first pair of plastic substrates have an opticalretardation with less than ±20% variation in uniformity for any givenwavelength of incident light; the mixture further comprising an ionicelement to induce dynamic scattering in the liquid crystal host when asecond voltage is applied to the cell; wherein the optical deviceexhibits a wide absorption band having an aggregate full width at halfmaximum (A-FWHF) that is greater than 175 nm within a visible wavelengthrange of 400-700 nm; wherein the optical device has a dark statetransmission equal to or below 40% and a transmission swing between theclear state orientation and the dark state orientation greater than orequal to 30%.
 19. The optical device of claim 18, wherein saidguest-host mixture has a clear state orientation when no voltage isapplied to the cell and a dark state orientation when a voltage isapplied to the cell, and wherein said liquid crystal host comprises achiral nematic material with a thickness to pitch ratio (d/p) of lessthan 0.9 but greater than or equal to 0.25.
 20. The optical device ofclaim 18, wherein said guest-host mixture has a dark state orientationwhen no voltage is applied to the cell and a clear state orientationwhen a voltage is applied to the cell and wherein said liquid crystalhost comprises a chiral nematic material with a thickness to pitch ratio(d/p) of less than 5 but greater than or equal to 1.75.